1. Field of the Invention
The present invention relates generally to the field of subsea electrical power cables. More specifically, it relates to a system for low voltage direct electrical heating (DEH) of a coaxial pipe or riser.
2. Description of the Related Art
The temperature of the oil or condensate in the underground reservoirs is typically about 90° C. The oil or condensate well stream contains several liquid substances that freeze when the temperature drops. This is a problem when the pipes are cooled in seawater, particularly during a shutdown of production, which causes the flow in the line to be impeded or even blocked due to the formation of hydrates or wax plugs. To solve this problem chemical treatments are mainly used. However, this has considerable operational costs and presents a risk to the environment should a leakage occur. Chemicals are used to prevent hydrate or wax formation during production shut-downs.
As an alternative to chemical treatment, hot water lines and electric heating has been in use for some years. Early ideas on how to extend DEH to include the production riser are all focused on utilizing one and the same electrical current, which invariably results in the need for high-voltage (HV) insulation in the riser.
Indirect heating using hot water or electrical heating cables will not dissipate heat at the optimal position within the riser. In addition, hot water will loose temperature with increasing depth and distance to a topside host. Use of chemicals is generally unwanted, mainly due to logistics and cost. Series connection with an existing DEH system, using a piggyback cable, would result in a need for high-voltage insulation, and also introduce uncontrolled current flow from sea water to topside structure.
In a paper, ISOPE 2000 (Halvorsen, Lervik and Klevjer), are described some different DEH-concepts. Three DEH-concepts are illustrated in FIG. 4 of said paper (not attached here) where:
a) is a common DEH-concept (DEH-a) for heating of a flowline and where the riser is excluded from the DEH. This concept is how DEH is utilized today. In operation, a current flows from a 50 (or 60) Hz alternating current (AC) power source on a platform, down to an isolated forward cable from the platform further down to and along the pipeline laying on the sea bed fastened to the pipeline as a piggyback cable and grounded at a far end. The current flows through this cable and returns via the pipeline to the point where the riser is connected to the pipeline. From this point the current is led through a separate return cable up and back again to the power source on the platform. Because the pipeline is grounded at both ends, a parallel current is flowing in the water surrounding the pipeline on the sea bed.b) is a DEH-concept (DEH-b) where DEH is extended to cover the pipeline as well as the pipeline on the sea bottom. A current flows from a 50 (or 60) Hz AC power source on a platform, along a piggyback cable downwards along the riser and further along the pipeline. The piggyback cable is connected to the pipeline and to ground at a far end. The current returns via the pipeline and further on to the end of the pipe in the riser. At the platform, the current is led back again to the power source on the platform. Because the pipeline and the pipe in the riser are grounded at their ends, parallel currents are in this configuration flowing in the water surrounding the pipeline and surrounding the pipe in the riser. Electrical current flowing in the pipe in the riser will generally be different from the current flowing in the pipeline on the sea bottom, with the current difference flowing in sea water.c) is a DEH-concept (DEH-c) where DEH is extended to cover the pipeline as well as the pipeline on the sea bottom like in DEH-c, but where the riser/flowline connection is insulated and not grounded at this point. This forces the same electrical current to flow in the pipe in the riser and in the pipeline on the sea bottom.
A problem with the DEH-a is that the last part of the pipeline e.g. the riser, umbilical or stiff pipe, is not getting heated by the DEH.
A problem with the DEH-b is that the high voltage and the resulting not well controlled current in the surrounding water are potential threats to safe operation. Depending on the construction, high-voltage (HV) insulation will probably be required in the riser. Said not well controlled current may potentially amount to several hundred amperes. DEH-b will therefore hardly get accepted for use in the oil industry. For dynamic/free-hanging risers, another problem is related to the need to attach the piggyback cable to the dynamic riser. A piggyback element like this will generally result in unpredictable dynamic behavior of the dynamic riser, and may adversely influence clashing and/or fatigue properties of the riser.
DEH-c has the same drawbacks regarding security as DEH-b, the only functional difference being that the electrical current flowing through the riser is the same as that flowing through the pipeline on the sea bottom. Whether the current to be transferred from sea water to production unit (offshore platform or onshore) is larger or smaller than for DEH-b depends mainly on differences in geometry and material properties between riser and pipeline on sea bottom.
A direct heating system is based on the fact that an electric current in a metal conductor generates heat due to ohmic losses. In US2002122664 to Nexans, a system for heating a flow line including an umbilical is presented. In this application, the pipeline itself is used to dissipate ohmic losses and therefore also acts as a heating element. A separate insulated conductor is used to supply forward current. In this system, the far end related to a production platform, is the ground reference which is in electrical contact with the sea. The other end is insulated from the sea. This results in the voltage difference between the production line and the sea is increasing when moving from the far end and towards the platform. A disadvantage with this system is that the voltage difference between the sea and the pipeline will result in the need for high voltage (HV) insulation at the platform and some distance along the pipeline.
GB2435347 and GB2437161, both to Nexans, describe two other systems for DEH which are based on separate piggyback cables. GB2457791, also to Nexans, introduces yet another system with a piggyback cable. This application describes a system where the pipeline and the piggyback cable are placed together inside a magnetic core.